[2][3] Chun Sung-geun (Kim Soo-ro) is the gym teacher of a private high school in an upscale district where competition is heated among students to enter top colleges.

by his students because of his quirky appearance and behavior, he is a teacher who has lived life his own lazy, ignorant way for years without any worries.

In danger of losing his position, he remembers by chance that he obtained a license to teach English a decade ago.

The school board president tells him to pass a test in order for her to acknowledge it.

His only weapon is his amazingly incomparable physical strength, and with the help of a few students who understand his warm heart, the simpleton Sung-geun begins his rigorous training to exercise his brain "muscles" to master English in his journey to become the school's E.T.